Willig visited the towers a year before the stunt and took measurements for the equipment he would need. He made special clamps that fit into the window washing tracks of the South Tower. The clamps he designed would lock into place when they were pulled down by his body weight. They would release when he decided to raise them. Once he built the equipment, he went to the World Trade Center 4 to 5 times at night to test the equipment. He began his climb at 6:30 a.m. that Thursday morning. As he was climbing, two police officers, one a suicide expert, were lowered down in a window washing basket to try to get Willig to give up. Willig swung away from the officers so they could not grab him. Willig and the officers talked, and the suicide expert realized that Willig knew what he was doing and was not a threat. The officer passed him a pen and paper, and Willig signed it "Best Wishes to my co-ascender." Police helped him to the top of the tower, by pulling him through a tiny window hatch at 10:05 a.m. and he was arrested. Willig said he could hear the crowd cheering from ground level. His climb received plenty of attention because it took 3½ hours to complete, allowing news cameras and spectators to gather. The only significant problem Willig ran into was irregularities in window washing tracks. However, he was prepared for this because he brought a small hammer to fix the irregularities. He signed his name and the date on a piece of metal on the observation deck of the South Tower, which was still visible until the tower was destroyed on September 11, 2001.

After the 9/11 attacks that destroyed both towers of the World Trade Center, Mr. Willig publicly said he regretted climbing the towers, as his actions may have brought them to the attention of terrorists. But he later told CNN that was just an initial, emotional reaction and that he was still glad to have climbed the towers.[1]